A switching power supply device of the related art has a configuration in which a pulse voltage is generated from an input voltage by a switching operation and an output voltage is obtained by smoothing the pulse voltage. In the switching power supply device, it is possible to obtain a desired positive voltage and a desired negative voltage. The efficiency of the obtained voltages is as high as 80% to 95%. Thus, the switching power supply device is superior from the viewpoint of power-saving design.
However, if a wireless communication unit for processing ultra-micro signals, such as Bluetooth, GPS (Global Positioning system) or WLAN (Wireless Local Area Network), exists in the vicinity of the switching power supply, communication is hindered by even a small amount of noise. This makes it impossible to perform basic operations, thereby causing a malfunction. Major causes of this phenomenon include PI (Power Integrity) noise, EMI (Electromagnetic Interference) and emission noise of the switching power supply.
In the related art, therefore, the switching power supply is used in equipment having no wireless communication unit. If a communication unit exists near the switching power supply, a filter or a choke coil for reducing PI noise or EMI noise is arranged in addition to the switching power supply. It is also known that the switching power supply is wholly surrounded by a tube shield to prevent the influence of a switching noise. In the related art, a noise-cancelling dummy switch is provided to cancel the electric fields between an output-voltage-generating switch node and a noise-cancelling switch node.
In the related art referred to above, however, the costs grow higher. Moreover, the substrate area, the volume and weight of the parts, the number of parts and the reliability work against the switching power supply and become an obstacle in designing a small, lightweight and high-performance product. In addition, the methods of the related art, such as the method of providing a filter or a choke coil for reducing PI noise or EMI noise in addition to the switching power supply, suffer from a problem in that it is difficult to sufficiently reduce noise. In order to obtain a positive voltage output and a negative voltage output, there is a need to separately provide a voltage boosting circuit and a voltage dropping circuit. If the noise-reducing parts stated above are added to the voltage boosting circuit and the voltage dropping circuit, the costs grow higher. The addition of the noise-reducing parts becomes an obstacle in designing a small, lightweight and high-performance product.